Dystregulated homologous recombination can lead to genomic instability, which is manifested by gene inversion, deletion, amplification, translocation, and loss of heterozygosity. Genomic instability can provide a driving force for the acquisition of multiple genetic alteractions and may lead to tumorigenesis. Eukaryotic topoisomerase III was first identified in yeast based on the phenotypes of the deletion mutant, which include hyper-recombination of repetitive sequences, slow growth, and defects in sporulation. The mutant also demonstrates late S/G2 delay in the cell cycle, shortened telomeres, and increased chromosomal losses. Topoisomerase III physically and functionally interacts with a yeast DNA helicase SGS1, which belongs to a family of proteins including the human Bloom's syndrome and Werner's syndrome gene products. The interaction between topoisomerase III and helicase is evoluationarily conserved. Mutation of SGS1 suppresses the topoisomerase III mutant phenotypes. The sgs1 mutant also exhibits premature cell aging, redistribution of SIR3 silencing protein complex from telomeres to nucleolus, and nucleolar fragmentation. A human homologue of yeast topoisomerase III, hTOP3a, has previously been isolated and suggested to be involved in genetic instability of ataxia telangiectasia cells. He has recently cloned the cDNAs for the alternative splice forms of a second human topoisomerase III, hTOP3b. He has also demonstrated that the largest gene product of hTOP3b can interact with the yeast SGS1 protein, and expression of hTOP3B can partially complement the slow growth phenotype of the yeast topoisomerase III mutant. In this proposal, he propose: 1) to compare the biochemical properties and biological functions of the human topoisomerase III proteins; 2) to determine if there are growth and genetic changes in the cell lines with enhanced or disrupted expression of hTOP3a and hTOP3b isoforms; and 3) to identify and characterize proteins which interact specifically with hTOP3a and hTOP3b. The results of these studies will provide significant information about the physiological roles of various human topoisomerase III gene products and their effects on maintaining genomic integrity. Identification of the interacting proteins and elucidation of the interaction mechanism may provide insights into how these proteins regulate genomic stability, cell growth, cell aging, and ultimately, cancer development.